Make rootless settings configurable

[brandond]

Proposed Changes

Add environment variables for port-driver, cidr, mtu, enable-ipv6, and disable-host-loopback settings. Since rootless is still experimental, I don't think they deserve full CLI flag status.

Types of Changes

new feature

Verification

Follow the rootless K3s setup instructions (delegating cgroup controllers), then create a k3s-rootless.service as follows:

[Unit]
Description=k3s (Rootless)

[Service]
Environment=PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin K3S_ROOTLESS_MTU=1500 K3S_ROOTLESS_PORT_DRIVER=slirp4netns
ExecStart=k3s server --rootless --snapshotter=fuse-overlayfs
ExecReload=/bin/kill -s HUP $MAINPID
TimeoutSec=0
RestartSec=2
Restart=always
StartLimitBurst=3
StartLimitInterval=60s
LimitNOFILE=infinity
LimitNPROC=infinity
LimitCORE=infinity
TasksMax=infinity
Delegate=yes
Type=simple
KillMode=mixed

Testing

tbd

Linked Issues

• rootless k3s: allow configuring the mtu of slirp4netns #6487
• Rootless: Add abillity to select port driver #6488

User-Facing Change

The rootless `port-driver`, `cidr`, `mtu`, `enable-ipv6`, and `disable-host-loopback` settings can now be configured via environment variables.

Further Comments

[nikolaishields]

LGTM